Coral reef with bright colors

A shallow reef in Kāneʻohe Bay during a warm-water episode. (Photo credit: Chris Wall)

Climate change and ocean warming threaten coral reefs and disrupt the harmonious relationship between corals and their symbiotic algae, a process known as “coral bleaching.” However, a new study, conducted by scientists at the University of Hawaiʻi at Mānoa and the California Academy of Sciences, revealed soft tissues that cover the rocky coral skeleton promote the recovery of corals following a bleaching event.

These soft tissues, which are home to beneficial algae, represent a source of energy for corals. The study, led by Chris Wall, a graduate student at the Hawaiʻi Institute of Marine Biology in the School of Ocean and Earth Science and Technology (SOEST), showed corals with thicker tissue may be better equipped to survive bleaching in a warming ocean.

Said Wall, “While we know a great deal about thermal stress and its effects on corals, we know comparatively little about how corals recover from bleaching in the real world, or how local factors, such as light or nutrients in seawater, can influence recovery from bleaching.”

How do corals overcome their traditional dilemma?

Pigmented coral on left and bleached coral on right

Finger coral, pigmented, left, and bleached, right. (Photo credit: Raphael Ritson-Williams)

In fall 2014, Wall and colleagues studied colonies of two species of corals, rice coral and finger coral, in Kāneʻohe Bay. Seawater in the bay reached the unusually high temperature of 86 degrees, which is near the maximum temperature Hawaiian corals can tolerate. The team was interested in how colonies that were sensitive to thermal stress responded to and recovered from bleaching compared to adjacent coral colonies that remained pigmented and did not bleach.

During the warming event and three months later, the team assessed the coral colonies and their symbiotic algae, and throughout the study measured environmental factors including light levels, water temperatures, sedimentation rates and seawater nutrients to better understand how environmental factors influenced the severity of coral bleaching and rates of recovery. The researchers also used naturally-occurring chemical signatures in coral tissues to test how corals were performing and what they were eating during and after stress.

“A coral’s diet is based on food from their symbionts and the consumption of small organisms in seawater known as plankton, and these two sources supply the building blocks for coral tissues. But under bleaching, corals are left without their symbionts and are in effect starving,” said Wall. “We wanted to know how corals overcome this nutritional dilemma—were they relying on stored energy in their tissues [like a bear in hibernation] or were they eating more plankton?”

Recovery hastened by environmental factors

The bleached colonies did not die and showed remarkable resilience, recovering from losses in both their symbionts and soft tissues within three months. This recovery was hastened by environmental factors, such as cooler water temperatures and water with low nutrient concentrations, which influenced coral tissues.

The researchers determined that coral tissues are very important in the bleaching recovery process and that corals with abundant or thick tissues may be able to better survive and recover from bleaching stress. The stored energy in coral tissues, and not greater plankton feeding, served as food for corals during thermal stress and helped corals recover.

For the full story, see the SOEST website.